In early November, the U.S. will put more sanctions on sales of Iranian oil. Some of Iran's oil customers, such as China, may be undeterred, as NPR's Jackie Northam reports.

JACKIE NORTHAM, BYLINE: President Trump has faced a lot of pushback, particularly from European nations, for pulling out of the Iranian nuclear deal and trying to prohibit Iran's oil exports in order to starve it for cash. And it's having an impact.

DIWAN: In particular, we've seen the Japanese going very quickly to zero, the Koreans reducing on their way to zero and the Europeans - most of them have already exited the market.

NORTHAM: Iran's oil sales have dropped more than 30 percent since April, the month before Trump announced plans to reimpose sanctions. Elizabeth Rosenberg, who worked on sanctions for several years at the Treasury Department, says countries are reducing their oil purchases because the U.S. has enormous leverage to place financial penalties on any company doing business with Iran.

ELIZABETH ROSENBERG: And for most international companies, banks, individuals, it is tantamount to closing up shop, announcing bankruptcy if you sever those ties from the U.S. dollar and the U.S. financial system.

NORTHAM: Rosenberg says that goes for energy companies, oil refiners and shippers and central banks in Europe and Asia. But the European Union wants to save the nuclear deal, and it wants to continue doing business with Iran. Rosenberg says the EU is trying to devise a special payment system to work around the U.S. sanctions.

ROSENBERG: It would be created for this purpose, and that means that it doesn't feature the greatest efficiency. It may have plenty of kinks because it's being worked out for the first time.

NORTHAM: Rosenberg says there are other ways of circumventing sanctions, involving sophisticated accounting tricks or putting payments in escrow. China has also signed the nuclear deal. It's Iran's biggest customer and could be looking at ways to insulate smaller Chinese banks from U.S. sanctions or use its big banks.

ROSENBERG: The calculus on the Chinese part may be, how do you play chicken with U.S. regulators? Do you funnel those transactions through your biggest banks, betting that they are too big to fail, too big to sanction, or do you funnel those transactions through small banks that are almost entirely out of the reach of U.S. jurisdiction?

NORTHAM: So far, the sale of Iranian crude to China has stayed steady. Erica Downs, with the Center for Global Energy Policy at Columbia University, expects China will start to reduce their purchases in a bid to show it's complying with the spirit of the sanctions.

ERICA DOWNS: That said, I would not expect any Chinese leaders or Chinese oil company executives to necessarily come out and say, oh, we're reducing our Iranian crude imports because we want to comply with the U.S. sanctions.

NORTHAM: The (Unintelligible) it wants Iranian crude sales down to zero. Diwan of IHS Markit says the administration better watch what it hopes for.

DIWAN: If Iranian export go to zero and these barrels cannot be replaced by the rest of the producers in the world, oil prices will go up.

NORTHAM: Meaning higher prices at the gas pump, something the Trump administration wants to avoid ahead of midterm elections which occur just as the sanctions are due to take effect. Jackie Northam, NPR News, Washington.